1. Field of the Invention
The present invention relates to a steel product with plating layers and, more particularly, to a steel product with plating layers suitable for use as automotive parts, such as pipes.
2. Description of the Related Art
Automotive pipes include, for example, fuel pipes, brake pipes, air conditioner pipes, power steering pipes, lubricating pipes and cooling water pipes. Pipes with plating layers (hereinafter referred to simply as "plated pipes") are used as those automotive pipes to meet the requirements for corrosion resistance and strength. As shown in FIG. 4, a known plated pipe 1 has a steel pipe 2, a Ni plating layer 3 as a lower inner layer formed on the outer surface of the steel pipe 2, a Zn--Ni alloy plating layer 4 as an outer layer formed on the Ni plating layer 3, and a chromate layer 5 as an outermost layer formed on the Zn--Ni alloy plating layer 4.
When this known plated pipe 1 is used in a corrosive environment for a long time, portions of the steel pipe 2 corresponding to pinholes in the Ni plating layer 3 are corroded and red rust is formed on those portions. As the portions are further corroded, cavities are formed by pitting in those portions of the steel pipe 2 on which rust is formed. Eventually, through holes are formed in the steel pipe 2 and, consequently, the plated pipe 1 looses its function.